Prior to being marketed, natural gas is subjected to three main operations: deacidizing, dehydration and gasoline extraction.
The purpose of the deacidizing operation is to remove the acid compounds such as carbon dioxide (CO2), hydrogen sulfide (H2S), carbonyl sulfide (COS) and mercaptans, mainly methylmercaptan, ethylmercaptan and propylmercaptans. The specifications generally allowed for deacidized gas are 2% CO2, 4 ppm H2S and 20 to 50 ppm total sulfur content.
The dehydration operation then allows to control the water content of the deacidized gas in relation to transport specifications.
Finally, the gasoline extraction operation allows to guarantee the hydrocarbon dew point of the natural gas, also according to transport specifications.
The deacidizing operation, which is essentially intended to reduce the CO2 and H2S content of the gas, is for example performed by means of an absorption method, using notably chemical solvents like, for example, alkanolamines such as diethanolamine (DEA) or methyldiethanolamine (MDEA). After this treatment, the gas meets the specifications relative to the CO2 content, typically below 2% by mole, and to the H2S content, typically 4 ppm by mole. Part of the light mercaptans, notably methylmercaptan, is removed during this operation. The heavier mercaptans such as ethyl, propyl and butylmercaptan, or containing more than four carbon atoms, are not soluble enough in an aqueous solution or acid enough to significantly react with the alkanolamines generally used for deacidizing, and a great proportion thereof therefore remains in the gas. Most of these acid compound absorption methods have a mercaptan extraction efficiency ranging between 40% and 60%. Some technical solutions using solvents with a high physical absorption capacity such as water-alkanolamine-sulfolane mixtures, which achieve 90% sulfur compound elimination but with a significant energy consumption, notably because of the high solvent flow rates required by such performances, can however be mentioned.
Methods only based on the use of a solvent characterized by a high acid compound physical absorption capacity would allow removal of the mercaptans. They are however often unsuited to the constraints involved for CO2 and H2S removal. The absorption capacity of these physical solvents is proportional to the partial pressure of the contaminants to be removed from the gaseous effluent. High flow rates and a costly solvent regeneration are then required for specifications to be met. Furthermore, these methods entail a risk of accumulation, in the solvent, of long carbon chain mercaptans, i.e. having more than three carbon atoms, notably because of the boiling point of these sulfur compounds that is sometimes higher than the boiling point of the physical solvent selected.
The dehydration operation can be carried out by means of a glycol process (for example the process described in document FR-2,740,468), using notably TEG, which allows to lower the water content of the gas down to a value close to 60 ppm by mole. The mercaptans are not eliminated in this stage. An adsorption method of T.S.A. (Thermal Swing Adsorption) type on molecular sieve, for example of 3A or 4A or 13X type, or on silica gel or alumina, can also be used. In this case, the water content of the gas is typically below 1 ppm by mole.
A last fractionation operation by cooling finally allows the treated gas to be separated into its different constituents so as to valorize each cut produced: C1 cut, C2 cut, or C1+C2 cut, C3 cut, C4 cut, and heavier C5+ cut, possibly further separated into various complementary fractions. The major part of the sulfur compounds is concentrated in the liquid phases, which therefore have to be processed later to meet the sulfur specifications generally required.
It is therefore necessary to carry out one or more additional processing stages, depending on the distribution of the mercaptans in the various cuts obtained after fractionation. According to the technology selected, this mercaptan removal stage takes place at various points of the natural gas processing chain.
Generally, the mercaptans are removed by caustic washing of the liquid hydrocarbon cuts obtained from fractionating. Countercurrent contacting, in a plate column, of the hydrocarbon feed with a concentrated soda solution, between 10% and 20% by weight, provides elimination of all the sulfur compounds such as COS and the mercaptans. The mercaptans react with the soda and give mercaptides, which are then oxidized in the presence of a catalyst present in the solvent and give disulfides, while regenerating the caustic solution. The latter are then separated by decantation of the aqueous phase. The efficiency of this technique is furthermore closely linked with the nature of the mercaptans to be removed: it decreases as the number of carbon atoms of the hydrocarbon chain of the mercaptan increases. This can be explained by the low solubility in an aqueous solution of mercaptans having more than three carbon atoms. Furthermore, the presence of COS reacting irreversibly with soda leads to a high base consumption.
An alternative to the caustic washing technique is elimination of the mercaptans upstream from the fractionation stage. This complementary treatment intended to lower the residual mercaptan content can consist of an adsorption stage using for example a 13X zeolite for desulfurization, the pore size of these zeolites allowing complete adsorption of all the mercaptans, including the biggest ones. The methods used are then T.S.A. (Thermal Swing Adsorption) type processes wherein adsorption takes place at ambient or moderate temperature, typically ranging between 20° C. and 60° C., and desorption at high temperature, typically between 200° C. and 350° C., under sweeping of a regeneration gas, which can notably be part of the purified gas, generally between 5% and 20% of the flow of feed gas. The regeneration gas is preferably recycled upstream from the glycol dehydration plant or upstream from the adsorption purification units. The pressure is either maintained substantially constant throughout the cycle, or lowered during the regeneration stage so as to favour regeneration. At the outlet of this adsorption purification stage, the gas meets the total sulfur specifications.
The regeneration gas containing a large amount of mercaptans must however be treated prior to being recycled, for example by washing with a basic solution (soda or potash), with known limitations due to the low solubility of mercaptans in an aqueous solution. One drawback of these adsorption sieves lies in the production of a mercaptan-rich gaseous effluent which also has to be processed. Current techniques for removing mercaptans from a gaseous effluent are often ineffective or economically unsuitable.
The present invention provides a new technique for removing the mercaptans contained in a natural gas. In general terms, the mercaptan-laden natural gas is contacted with a mercaptan-adsorbing sieve. The mercaptan-rich gaseous effluent obtained upon regeneration of the sieve is then contacted with an olefin-containing liquid feed in the presence of an acid catalyst. Under suitable conditions, the mercaptans are absorbed in the liquid feed and react with the olefins so as to form sulfides soluble in the solvent. A solvent regeneration stage allows the capture agent to be recycled.